High Cold Idle - Dodge Challenger Forum: Challenger & SRT8 Forums
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post #1 of 18 (permalink) Old 04-19-2019, 08:38 PM Thread Starter
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High Cold Idle

My 2017 is my first Challenger RT 6 speed. On cold start it’ll idle up over 1000 rpm for a while. A little hard on my clutch when I wanna go before it idles down. Usually about 6-9 minutes.

Do any of you know if that’s adjustable? Maybe seasonally so it doesn’t go that high in the warmer weather.
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post #2 of 18 (permalink) Old 04-19-2019, 09:13 PM
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Originally Posted by Morey View Post
My 2017 is my first Challenger RT 6 speed. On cold start it’ll idle up over 1000 rpm for a while. A little hard on my clutch when I wanna go before it idles down. Usually about 6-9 minutes.

Do any of you know if that’s adjustable? Maybe seasonally so it doesn’t go that high in the warmer weather.
The idle can be adjusted by HP tuners which requires an unlocked PCM and the tuner which will run you +$600 and void your warranty. There are several tables but the main ones are elevated and manual w/AC (seems to control final idle). They all have dependency on engine coolant temp but elevated is also time dependent.

Anyways, check to make sure you intake manifold bolts are torqued properly. On my 2015 RT M6 I notice fuel stains around the intake/heads and sure enough all the bolts where very loose. I forget the torque off hand but you can find it from the online chilton manual in this sticky.
https://www.challengertalk.com/forum...access-666787/

As a side note, the M6 uses a clutch delay valve which delays the engagement of the clutch when the pedal is released. I does this to reduce shock to the drivetrain. Most remove it as it improve the connectivity between the pedal and clutch. There are youtube vids showing how it is removed...~15min job.
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2015 Challenger RT+ (non-STP)
8lb Magnuson w/ Innovate PSB-1 wideband/boost/shift light gauge
Corsa extreme (2.5")
4" Aluminum one-piece driveshaft
Hellcat bottom airbox
Stainless Works LT headers (2" primaries)
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Comp Cam 7929 & 7748 push rods (gives slightly more lifter preload)
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post #3 of 18 (permalink) Old 04-20-2019, 07:02 AM Thread Starter
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Thanks Chally. I'll look into the tuner.
Are you talking about what I call the Hill Holder?
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post #4 of 18 (permalink) Old 04-20-2019, 07:24 AM
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Thanks Chally. I'll look into the tuner.
Are you talking about what I call the Hill Holder?
The clutch delay valve is different from "hill assist". The CDV delays the clutch from engaging...the hill assist delays the brakes from disengaging.
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post #5 of 18 (permalink) Old 04-20-2019, 08:40 AM
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With a newer car -- since at least 1996 when OBD2 came into the picture and fuel injection, more precise fueling both at warm and hot but also cold engine operation -- incorrect (too high, or too low) idle speed is rare.


With various cars over the years and started cold -- from a low of 0F degrees to up to 100F degrees -- the elevated idle speed after a cold start lasts at most nearly a minute from the coldest of starts.



Based on my observation the idle speed drops to near normal hot idle speed when the engine controller switches from open loop mode to closed loop mode, and concurrent with it shutting off the secondary air injection system.


The converters and the O2 sensors are hot enough to operate as they should and the engine controller can now rely upon the O2 sensors to manage fueling of the engine to ensure proper engine operation and proper converter operation.


A 6 to 9 minute elevated idle speed after a cold start is not normal. My thinking would not be to find a way to adjust this but find out why the idle speed remains so high for so long. Is the engine controller really having to run the engine at an elevated speed that long to warm up the O2 sensors and converters?



With a proper OBD2 tool one could monitor the engine controller and see what it switches from open loop mode to closed loop mode. There is a problem if the switch occurs in less than 6 to 9 minutes but the engine speed remains elevated or if it takes 6 to 9 minutes for the switch from open loop mode to closed loop mode to happen.


I can't believe a 6 to 9 minute fast idle is necessary to warm up the sensors and converters unless there is something possibly interfering with the air/fuel ratio that results in a much leaner mixture. Lean in the sense the extra fuel injected when the engine is cold and intended to be burned at the converters to speed their warm up is being leaned out and burned in the engine.


Thus the recommendation to check the intake manifold bolts for proper tightness is I think a good suggestion.


With an OBD2 tool you can also check/monitor the intake air temperature and coolant temperature. In some cases when there is a problem with one of these sensors and it reports the wrong temperature this can affect engine behavior at engine start or at any other time.
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post #6 of 18 (permalink) Old 04-20-2019, 11:39 AM
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Like I said and showed in my previous post, the PCM controls elevated idle based off engine coolant temp and engine run time...doesn't rely on open/closed loop conditions...engine temp and run time. Closed loop fueling on these cars start at surprisingly cool temperatures when ECT is ~5F and the rear O2s are at 50F.

One thing that could be the cause for long high idle is a thermostat that is stuck open, but that would usually lead to a DTC, very common when guys in the northern states swap the thermostat to 180F instead of the stock 203F.

Other than that, like I said and have seen with other M6 5.7s, the intake manifold bolts could be loose and grabbing false air. What is surprising with these motors is that if you pull the evap purge hose the engine will not throw a DTC but will adjust rpms through throttle/spark to compensate for the increased flow. Not sure how the 2015-2016 hell cats handle this as they also run a MAF, but all the other HEMIs don't seem to be terribly sensitive to vacuum leaks. On my 2013 JGC I noticed the idle was kind of funky, only to find the evap purge hose completely crack off at the throttle body. No DTC showed up and I only looked into the problem because the idle seemed rough when the engine was hot.
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post #7 of 18 (permalink) Old 04-21-2019, 09:39 AM
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While I believe you I still find it hard to believe Dodge maintains a high "cold" idle based on coolant temperature and run time. I wonder for what purpose? (This also really calls into the question the appropriateness, as if there wasn't sufficient question to begin with, the installation of a low temperature T-stat.)

At cold start, the goal for other automakers is to run the engine at an elevated idle speed only long enough to get the O2 sensors and converters up to operating temperature and then as soon as this happens to concurrently drop the idle speed to near hot idle normal speed and switch to closed loop mode to more precisely fuel the engine, for better driveability and lower emissions and better fuel economy.

Need to get out my Auto Xray OBD2 tool and monitor when my Hellcat switches to closed loop mode.
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post #8 of 18 (permalink) Old 04-21-2019, 10:47 AM
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Originally Posted by Rockster View Post
While I believe you I still find it hard to believe Dodge maintains a high "cold" idle based on coolant temperature and run time. I wonder for what purpose? (This also really calls into the question the appropriateness, as if there wasn't sufficient question to begin with, the installation of a low temperature T-stat.)

At cold start, the goal for other automakers is to run the engine at an elevated idle speed only long enough to get the O2 sensors and converters up to operating temperature and then as soon as this happens to concurrently drop the idle speed to near hot idle normal speed and switch to closed loop mode to more precisely fuel the engine, for better driveability and lower emissions and better fuel economy.

Need to get out my Auto Xray OBD2 tool and monitor when my Hellcat switches to closed loop mode.
Stock tunes do not maintain high idle for very long...should be under a couple of minutes and it gradually bumps down until it reach the lowest idle in the manual w/AC table...which is why I suspect the OP has something else going on. I looked through some of my start-up logs but none have the channel logged which states when closed loop occurs. I would check now but the car has already been started...maybe tomorrow morning on my way to work. On a side note, I do notice the evap purge operating at cold, high idle and at idle the VVT cam is fully advanced...nothing you can do about it either.

Ah, forgot STFT can be used as a CL indicator. Here is an example of a cold start...note STFT kicks in rather quickly.
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post #9 of 18 (permalink) Old 04-21-2019, 12:32 PM
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Dug out my Auto Xray tool and got it connected to the car's engine controller without having to start the engine first. Two presses of the start button with my foot *OFF* the brake does it. The tool scanned protocols and connected via CAN in nearly no time.

Then I called up all the data in data streaming mode. Forgot how many data items (PIDs) the Auto Xray can obtain/display compared to my old Actron tool.

Started the engine and noted of course the engine RPMs zooming up to just over 1K. Scrolled down and found Fuel System 1 and confirmed it was in open loop mode. To my surprise the engine didn't idle very long at all, "way" under a minute, before Fuel System 1 switched to closed loop mode.

I also noticed the engine RPMs didn't drop quite as much as I had expected but they did drop and were under 1K very quickly after the Fuel System 1 went to closed loop mode. Scrolled down and came to converter temperatures. Both at 1049F. Those suckers get plenty hot plenty quick.

As I continued to monitor the RPMs in 2 minutes the engine speed while still above normal hot idle speed was much closer to normal hot idle speed than not.

Granted the drop in cold idle RPMs is not as dramatic as it was with say my 996 Turbo Porsche but the Hellcat engine RPMs did fall and in ~2 minutes were nearly at the normal hot idle engine RPMs.

But the initial drop off was concurrent with the engine controller Fuel System 1 switching from open to closed mode. (BTW, there is a Fuel System 2 but it is not active on the Hellcat engine. Some engines have *2* controllers (the BMW 850i with the V12 engine -- treated like two 6 cylinder engines each with its own "engine" controller (which is one reason why when I had a chance to buy a very nice 850i I declined in spite of the fact I would have loved to have a car with V12 engine coupled to a 6-speed manual).


We are definitely in agreement the OP's car is not behaving normally.
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post #10 of 18 (permalink) Old 04-21-2019, 12:51 PM
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I know that my 2018 GT (3.6L V6) stays in somewhat high idle for MUCH longer than 2 mintues. I'm talking right about 900 rpm'ish. If I put it into drive when at 900 rpm, it'll immediately drop down to normal idle. But it will stay it that somewhat high idle for way longer than 2 minutes.

But that may be because it's a 6 cylinder and instead of an 8 cylinder... :-)
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